The cerebral metabolic responses to perinatal hypoglycemia were determined in newborn dogs by direct analysis of brain substrates, glycolytic intermediates and high-energy phosphate reserves. In addition, the characteristics of lactic acid transport into and metabolism by the perinatal brain during normoglycemia and hypoglycemia were further elucidated. Hypoglycemia, with final blood glucose values ranging from 0.1 to 1.5 mM/1, was associated with changes in cerebral metabolites which reflect reduced cerebral glucose consumption and glycolytic flux. Despite the alterations in carbohydrate metabolism, high-energy phosphate reserves are well-preserved even at the extreme of hypoglycemia. These data, coupled with a previous finding of enhanced lactic acid entry into and consumption by newborn dog brain, suggest that this metabolite serves as an important, if not the predominant, substitute fuel for cerebral oxidative metabolism during perinatal hypoglycemia. Further studies demonstrate that lactate flux into the perinatal brain appears to be concentration-dependent, consistent with simple diffusion at arterial lactate levels up to 8 mM/1. Uptake of lactate into brain appears to be enhanced during hypoglycemia, especially at high arterial lactate levels. These findings may have clinical relevance in human infants during hypoglycemia when lactate concentrations approach or exceed those of glucose.